This paper presents an optical vibration sensing system for internal arc testing of oil-immersed power equipment based on linear Sagnac unbalanced interferometric. The proposed system utilizes optical fiber sensing technology, which offers advantages such as resistance to electromagnetic interference, high sensitivity, compact size, and distributed sensing capability. The system employs a novel interferometric structure that replaces the traditional ring Sagnac interferometric structure with a Faraday rotating mirror for end-face reflection. This novel structure ensures high sensitivity, eliminates environmental interference, and improves the system's signal-to-noise ratio, making it suitable for the challenging arc testing of oil-immersed power equipment. Experimental results demonstrate the system's capability to accurately measure vibration frequencies within the range of low frequency to 6000 Hz, meeting the requirements of the transformer arc test process. Furthermore, the sensor is capable of measuring vibration accelerations up to 12 g, ensuring a wide detection range for vibration in the transformer arc experiment. These findings highlight the unique value and important contribution of the proposed optical vibration sensing system in the field of oil-immersed power equipment testing and evaluation.